Macro to bind proc to JavaScript method

bindMethodMacro.nim

import macros

# Macro to build a lambda using JavaScript's `this`
# from a proc, `this` being the first argument.
macro bindMethod*(procedure: typed): auto =
  var
    rawProc = getImpl(procedure.symbol)
    args = rawProc[3]
    thisType = args[1][1]
    params = newNimNode(nnkFormalParams).add(args[0])
    body = newNimNode(nnkLambda)
    this = newIdentNode("this")
    # construct the `this` parameter:
    thisQuote = quote do:
      var `this` {. nodecl, importc .} : `thisType`
    call = newNimNode(nnkCall).add(rawProc[0], thisQuote[0][0][0][0])
  # construct the procedure call inside the method
  if args.len > 2:
    for idx in 2..args.len-1:
      params.add(args[idx])
      call.add(args[idx][0])
  body.add(
    newNimNode(nnkEmpty)
  ).add(rawProc[1]).add(rawProc[2]).add(
    params
  ).add(rawProc[4]).add(rawProc[5]).add(
    newNimNode(nnkStmtList).add(
      thisQuote[0],
      newNimNode(nnkAsgn).add(
        newIdentNode("result"),
        call
      )
    )
  )
  result = body


# Usage example
type Foo = object
  # A method
  mt {.exportc.}: proc(a: string): cstring
  x: int

proc testMethod(a: Foo, b: string): cstring =
  cstring($a & b)

var test: Foo
test.mt = bindMethod(testMethod)
test.x = 42